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An exemption for New Zealand was now unlikely, but he and his ministry remained in contact with their US counterparts.

"Our trade officials in Washington and in New Zealand are trying to get to the bottom of what principle lies behind the decisions, but we've got no further information at this stage," Parker said.

He has noted that the US is considering quota limits on steel and aluminium for some countries, and said that could breach WTO rules.

A handful of countries have secured an agreement or entered negotiations with the US on exemptions from the tarrifs. New Zealand's omission from the list of countries went against the positive bilateral relationship with the US, Parker said.

He was challenged by former Trade Minister and National MP Todd McClay in the House today on why the EU, Canada, Mexico, South Korea, Argentina, Brazil and Australia had all received exemptions but not New Zealand. McClay also asked why Parker had not travelled to Washington to lobby the Administration, as his Australian counterpart had.

"I'm flattered that the member thinks that I could achieve an exemption by visiting when [Japanese] Prime Minister Abe visited the United States last week and was unable to get one."

He said he was advised against a visit by officials, who said it would not make a difference.

The impact of the tariffs on New Zealand's exports is relatively small - steel and aluminium make up about 1 per cent of total exports to the US. But the Government is concerned that the tariffs are further symptoms of rising protectionism and a breakdown of rules-based trade, which would have a disproportionate impact on a small country like New Zealand.

Earlier today, the Ministry of Foreign Affairs and Trade's chief negotiator Vangelis Vitalis said there had been a 30 per cent growth in protectionist measures in the last five years – the greatest increase since the establishment of the WTO 25 years ago.

"We are struggling to make progress multilaterally," he told the Foreign Affairs and Trade Committee at Parliament this morning.

"We have made some progress through the WTO, the trade facilitation agreement in 2013, and the elimination of agricultural export subsidies in 2015.

"But since then we have not made progress in the multilateral sphere, and that too underlines the importance of this mega-trade agreement [CPTPP] with its own set of rules and enforceable mechanisms that complement the WTO."

The 11-country CPTPP agreement was a hugely important part of New Zealand's defence against rising protectionism, Vitalis said.

He also warned that the broader safety net for global trade was under threat. The US has been blocking the appointment of new judges to the WTO dispute resolution body, and as a result it has shrunk from seven judges to four.

Parker said the US vetoes were of huge concern to New Zealand, which depends on the WTO to settle its trade disputes and protect its exporters.

"The advice that I've received is that if that continues, within about 18 months the WTO appellate body stops functioning."